Shutterstock is good but considering their pricing structure, it’s probably only useful if you need quite a few photos quite often, not just the occasional photo.

For one-off public domain images, I would look at the following free sources, many of which are searchable by topic (the first links to a list of sources):

http://www.freestockimages.net/resource-list/
http://www.publicdomainpictures.net/
http://www.public-domain-photos.com/
http://www.sxc.hu

The commercial stills issue is complicated I’m afraid. All comercials and TV shows are copyrighted and the to be totally safe, you should ask for permission from both the producer of the show, or advertiser and the agency who made the advert (which might be tricky to find out). However, seeing as you are using only one still from a 30 second (give or take!) advert or a longer TV show, it may be that your use can fall under the “fair use” exception where you are allowed to use a small portion of a copyrighted work for illustrative purposes. Unless you are planning to use large numbers of stills on a regular basis, I would say that trying to track down and get a response from the copyright owner might be more trouble than it is worth. The pragmatic approach would be to post the still and mention the show and the owner (e.g. BBC Television) and leave it at that. If somebody is aggrieved by your posting, then they will contact you – and in that case you must take the image down as soon as requested. But I honestly doubt that, if the clips are knocking about undisturbed on YouTube, anybody will pursue you for copyright infringement.

Hope that answers your question!